1. Field of the Invention
The present invention relates to particle beam optical apparatus and particularly to particle beam apparatus in which the evacuated housing is divided into two sequential zones of different pressure along the beam axis, each zone having a separate pump-out connection.
2. Description of the Prior Art
In some types of particle beam apparatus, the beam generating elements may be separated vacuum-wise from other parts of the apparatus. For example, field emission cathode beam generators can be operated only under ultrahigh vacuum (from about 10.sup.-9 to 10.sup.-10 Torr), while a high vacuum environment (from about 10.sup.-6 to 10.sup.-7 Torr) is sufficient for the other parts of the apparatus.
A specific example of a scanning electron microscope having a field emission cathode is described in a paper entitled "The Development of a Field Emission Scanning Electron Microscope" in the Proceedings of the Sixth Annual Scanning Electron Microscope IIT Research Institute, Chicago, April 1973. The equipment described in this paper has a partition that divides the beam housing transversely into an ultrahigh vacuum cathode chamber that contains the beam generating elements and a separate specimen chamber that is maintained at a higher absolute pressure. A disk valve for closing off the two chambers vacuum tight against each other is arranged so that it can be put in place from the lower pressure (i.e. the cathode chamber) side. To replace the valve requires manipulation within the cathode chamber, however, which involves the risk of contaminating this chamber.
The conduits leading from the separate vacuum subspaces or chambers of such apparatus should have as large inside diameters as possible because of the very low evacuation pressures that must be achieved. Conduits of such size, however, create mechanical instability or distortion of the beam housing, which may lead to difficulty in maintaining adjustment of the cathode in relation to the other parts of the equipment.
Furthermore, the vacuum conduits of all known dual subspace equipment of this type are spaced a considerable distance axially along the beam housing from each other, detracting from the overall appearance of the equipment. Also, the vacuum related components, such as the vacuum conduits, as well as the electron-optical elements cannot be mounted in the beam housing simultaneously. Because of space limitations the work must normally proceed sequentially and this means a long construction time.